In U.S. Pat. No. 5,631,815, the inventor in common to the subject invention describes a high voltage power supply in which a high voltage DC output is generated by magnetically coupling energy from a high frequency alternating electrical source to an arrangement of rectifier modules in the output stage. According to that invention, the rectifier modules and the associated magnetic coupling operate to limit the alternating voltages produced in the secondary windings to a level below the Paschen Minimum Voltage for the medium surrounding the rectifiers. Such a system facilitates the use of high frequency alternating voltages as the excitation source for the magnetic structure and eliminates partial discharges which are a serious insulation problem with alternating circuits at high voltage. High frequency excitation of the magnetic structure is desirable because it reduces the size of the structure required and similarly reduces the cost.
The High Voltage Power supply according to U.S. Pat. No. 5,631,815 uses an addition of many low voltages, which each are less than Paschen minimum or approximately 380 Volts, and as a result it is possible to use surface mount technology to construct the rectifier stages and to use printed circuit boards as carriers for the secondary windings and rectifier stages. Advantageously, this arrangement provides a reliable and low cost assembly suitable for operation at high frequency excitation. The modular nature of the arrangement makes it is possible to stack a large number of the voltage generating printed circuit boards to provide very high voltages. Provided that all printed circuit boards used in the supply are identical it is economical to create very high voltages by using a large number of the voltage generating boards connected in series.
In the power supply configurations discussed in U.S. Pat. No. 5,631,815 for the High Voltage Power Supply patent, the energy from the excitation source is provided to the secondary windings and rectifiers through magnetic coupling in a magnetic circuit or core. The magnetic core is assumed to be at ground potential and the voltage generating boards are insulated from the magnetic core. In practice that is a good design for moderately high voltages, say up to 200 kV. For higher voltages it becomes impractical to insulate the secondary windings of the supply from a grounded magnetic core at the high voltage end. This problem was encountered many years ago with high voltage power supplies excited with 60 Hz oscillations. The difficulty was overcome by segmenting the magnetic core and insulating the segments from each other with a polymer layer. The magnetic core segments were then maintained at a voltage level close to that of the neighbouring secondaries. Such a device, known as an Isolated Core Transformer or ICT was patented by Van der Graaf in the United States circa 1948.
It well known that even at 60 Hz there are problems associated with the use of an isolated core transformer especially at high currents. The segmentation of the magnetic core in the transformer introduces gaps in the magnetic structure with a permeability essentially that of air. This greatly increases the reluctance of the magnetic structure and produces leakage of magnetic flux. As a result the upper sections of the magnetic core carry less flux than the lower sections of the core, which results in a lower generated voltage per turn on the secondary windings. The leakage flux also gives rise to "leakage Reactance" in the equivalent circuit of the transformer. The leakage Reactance produces a drop in output voltage proportional to the load current. As a result of this leakage, an Isolated Core Transformer will exhibit a severe drop in output voltage with load current, i.e. voltage droop, and it is difficult to design an Isolated Core Transformer for very high voltages because of the loss of magnetic flux in the upper stages.
These inherent problems with Isolated Core Transformers operating at 60 Hz were addressed by Van der Graaf in the late 1940's and early 1950's. Van der Graaf overcame the problem to some extent by winding more turns on upper secondary coils. While this approach may be suitable for conventional ICT's, it is undesirable for the arrangement of voltage generating printed circuit board devices using surface mount technology in the high voltage power supply described in U.S. Pat. No. 5,631,815. One of the advantages of the high voltage power supply is the arrangement of identical voltage generating printed circuit boards for the output stages. The arrangement of identical voltage generating PCB's simplifies manufacturing and introduces economies of scale. Furthermore, the voltage generating PCB's result in an efficient utilization of the magnetic core structure. As a result, the application of Van der Graaf's techniques to the high voltage power supply would negate the inherent advantages of the design.
Another known approach in the art for addressing this problem is the "secondary excitation" technique. The secondary excitation approach involves providing an additional coil, similar in character to the primary, which is wound on the upper arm of the magnetic structure. The coil is connected to a capacitor to form a series resonant circuit with the coil. The value of the capacitor is selected to produce a resonant frequency close to the 60 Hz excitation. This technique is also referred to in the art as a "double tuning".
The "double tuning" approach is also unsuitable for the high voltage power supply described in U.S. Pat. No. 5,631,815. For high power high frequency operation of a DC power supply it is usually necessary to provide the AC input from a resonant inverter that converts the power of a low voltage DC bus into oscillations around 100 kHz. If the magnetic structure or the secondary circuits have a resonance close to that of the inverter, then control and stable operation is compromised. For this reason the conventional concept of double tuning is not suitable for the high voltage power supply.
In view of the foregoing, there remains a need for a unique arrangement for the high voltage power supply of U.S. Pat. No. 5,631,815, which provides an isolated core magnetic circuit without the inherent problems associated with leakage reluctance while maintaining the modularity of the output stages and providing the capability to achieve output voltages in excess of 200 kV DC.